I need to replace the batteries in the 2nd bank on my Catalina 34. As purchased used, it came with four 6v batteries wired in series/parallel. What are the pros/cons of replacing as is, or switching to four 12v batteries wired in parallel?

Well it depends on the quality of the batteries used. Sounds like these four 6v batteries are your house bank. Are they Trojan T-105s or similar? Golf cart batteries are well-suited to house bank, deep cycle use.

I would keep the same battery type unless you'd like to switch from wet-cells to some kind of newer gel, agm or whatever. Otherwise you're going to need to make up new batterycables and maybe rearrange your battery trays to fit some kind of 12v.

I have an RV, an interim vehicle until I find my dream boat, and this is a hot topic on RV forums as well. Personally, I have two 12 volt batteries wired in parallel, and they have provided ample power for my needs. However, the general gist of discussions I've read is that using 6 volt deep cycle batteries, i.e. golf cart batteries, will provide a stronger battery with longer life for the same amount of money spent on comparable 12 volt batteries.

6v any day. More amp hrs/$ and longer life.
Have both of my big boats house banks fitted for 6V. 12V for starter only.
Never run out of power yet.
Though as with any bank don't let them get below 50% as that shortens life.

Interstate does not make any batteries, they are a marketing/distribution brand, also supply Costco. I believe all their FLA are made by Johnson Controls. Not saying "bad", but the Duracell/Deka are more robust and last longer, better value for money. Then Trojans, a step up but a bit pricier.

You can keep going up the ladder, to Rolls / Surrette at the top.

With the top-notch vendors like that, they can be trusted to properly differentiate their "true deep cycle" from "dual purpose" starters, even in 12V models.

But they are more expensive, a huge step up in quality from what most people are willing to pay for.

So, it's not that 6V (or 2V, or 3, 4) is inherently better than 12V, it's just that at the bottom of the market, within consumer and automotive retail channels, the vendors all lie, and only the 6V FLA golf car batts are actually designed for deep cycling.